In order to investigate the radiative properties of plasma in non local thermodynamic equilibrium (NLTE), it is of great importance to determine energy level populations, which are often obtained by the so-called collisional-radiative (CR) model. As is well known, the CR model is accurate but computationally costly, and thus it is difficult to be applied to engineering calculations for such as complex three-dimensional plasmas.

In this work, a bound-state characteristic temperature (BCT) method is proposed, which can be used to calculate quickly the energy level populations in non-equilibrium plasmas. In this method, we assume that for each kind of ionization stage, the bound-state population is Boltzmannian at a certain characteristic temperature. The assumed characteristic temperature is related to the degree of none-equilibrium and may be different from the electronic temperature of the plasma. Based on a modified Saha equation, the assumed characteristic temperature can be calculated easily, and then the energy level populations are obtained conveniently. Five cases of non-equilibrium neon plasma at variable electronic temperatures and densities are investigated and compared with the results from a CR model. Good agreement is found between them if the degree of non-equilibrium is not very large. It shows that the present method is effective and at least 3000 times faster in computation time than the CR model. The method is very useful in engineering applications.